Hi Lexie,
Thanks for your comment. First, I would urge your friend to consider coming out to her parents about her self-harm if that's at all possible—I know for many people, this can help alleviate an otherwise pretty heavy secret to bear. However, I do understand that in some cases, this may do more harm than good—I will have to leave that up to your friend's judgment.
If disclosure is not an option, could your friend perhaps try arm bands? I've seen some quite pretty ones made to be worn on your upper arm—looking for adjustable ones might help to ensure they fit well and don't slide down during the day. Many of the ones I've seen are plastic or metal, but elastic armbands are also possibly an option. Temporary tattoos or body art to cover the area might also work, depending on whether she can wear anything like that at all on her skin.
I hope that helps. If you have more questions, comments, or concerns, feel free to leave a comment here or elsewhere on the blog. I'll be reading.
Sincerely,
Kim